This invention relates to telephonic communications with divers, and more particularly to improvements in systems for conversing with divers, notably those using diving rigs having a demand type of breathing gas supply.
Telephonic communication with divers has generally been marginal, at best, due to a number of factors including, as one of the most troublesome, the high level to noise produced in the diver's helmet or mask by incoming air or breathing gas. This is especially the case in demand breathing systems wherein a valve is actuated in response to incipient inhalation efforts to admit a burst of pressurized breathing gas into the helmet or mask, and to terminate the inflow of gas when inhalation ceases and exhalation is to begin. Because the demand valve operates intermittently, the breathing gas must be supplied at a greater flow rate than in continuous flow systems, a hissing noise is made that is of a high level during inhalation and that is absent during exhalation, or while the diver is speaking.
When more than one demand breathing diver is being monitored, or when two or more such divers are capable of communicating with one another and with monitoring tenders, the transmitted pulses of air flow noise during their respective inhalations interfere greatly with the intelligibility of their transmitted speech. At times the breathing of several divers will produce a substantially continuous high noise level at the receiving stations, and communications between divers and tending personnel are severly hampered.
Although the intermittent high noise level of demand air flow, generally referred to as inhalation noise, is deleterious to voice communication, it provides useful information to the diver tenders who monitor those sounds for clues as to the diver's condition. Accordingly, it is not desirable to eliminate the breathing or inhalation noises completely as by using a voice controlled microphone system that transmits only in response to predetermined human voice factors. Rather, both the voice communication and the breathing noise must be considered as useful communication data, and the problem is to monitor several divers without having the inhalation noises interfere with the voice transmission.
Various arrangements of automatic gain contrl amplification, selective frequency filtering, and thresholding techniques have been unsuccessfully tried in efforts to overcome this problem in diving telephony in the way background noise problems have been more less successfully treated in other situations. The primary reason for such lack of success is that the noise source in the diving system is located in a very confined area, such as the popular oronasal face mask known as a "Bankmask," wherein the microphone is substantially the same distance from the noise source as it is from the diver's mouth. This is in distinction with other noisy environments, such as factories or aircraft cabins, wherein the noise sources are at a distance from the microphone that is many times the distance between the speaker and the microphone. That distance disparity assures differences in sound energy levels that permit ready discrimination against distant background noise by the aforementioned prior art techniques, but do not discriminate effectively against inhalation noise generated in close proximity to the microphone of a diving rig.